Pharmacological study of the central analgesics generally involves two groups of experimental methods; one is based on the anti-nociceptive effects of the substances concerned and the other on the extent to which they are liable to produce tolerance and dependence.

In recent decades, the first group seems to have interested research workers far less than the second. Unless we are mistaken, the last important work on experimental evaluation of analgesic action to appear in this Bulletin
[
1] dates back to 1956, when Radouco-Thomas and his colleagues prepared an exhaustive bibliography on the subject, noting the application of mechanical, thermal and electrical stimuli at the level of pain receptors to reproduce the classic types of pathological pain (superficial, deep and visceral) in animals. Conversely, there has been a considerable increase in experimental models relating to psychotropic drugs; despite significant progress, exchanges between pharmacologists and clinicians is still the main method
[
2] , and there is no need to discuss it here.

Of the second group, we shall cite as examples the recently published remarkable, work of the Committee on Problems of Drug Dependence of the United States National Academy of Sciences-National Research Council
[
3] and that of Yanagita
[
4] published in this
Bulletin in 1973. WHO expert committees often base themselves on these methods when proposing classifications for new drugs. Schuster and Johanson
[
5] have recently published a didactic survey of them.

In thinking about the problem we wondered whether it would not be possible to tackle it from another angle. It is recognised that analgesics may act either peripherally (for example, acetylsalicylic acid) or at the level of the central nervous system (for example, morphinic substances). Why not look at this level for possible involvement of narcotic analgesics and other agents which induce certain peripheral activities by their impact on the central nervous system? A particularly fruitful area for research into effects of the central nervous system is the alimentary canal.

As experimental model, we selected external pancreatic secretion, which lends itself rather easily to quantifiable and reproducible measurements.

Since Pavlov's famous work, everyone knows that an imaginary meal increases the secretion of digestive juices. Similarly, administration of insulin or 2-desoxy-D-glucose induces digestive hypersecretion. This effect results from the action of a stimulus on the brain and is produced by the vagal nerves.

This action is known as the cephalic phase of gastric or pancreatic secretion.

The concept of the autonomy of the vegetative system is now outmoded. The workings of the "autonomic nervous system" (sympathetic and parasympathetic) and of the "central nervous system" are closely connected.

Parasympathetically, the pneumogastric nerves are afferent and efferent nerves conducting impulses in both directions. The vagal efferents, with which we are here concerned, spring from two main nuclei, the "
nucleus ambiguus", whose fibres innervate the laryngopharyngeal area, and the "dorsal nucleus, vagal motor", which is situated in the middle of the bulbus under the
fossa rhomboides and whose efferent fibres constitute the preganglionic fibres of the abdominal parasympathetic system. According to classic works on anatomy, the present knowledge relevant to an understanding of our problems may be summarized as follows: The observations of Fulton (1947-1949) and Hess (1948-1949) divide the vegetative centres of the
hypothalamus physiologically into: predominantly parasympathetic nuclei (supra-optic and paraventricular, main nuclei of the tuber), grouped anteriorly; predominantely orthosympathetic nuclei (lateral nuclei of the tuber, of
thearealateralis, posterior nucleus of the hypothalamus, nucleus of the mamillary tubercle) situated posteriorly.

According to Dell
etal. (1952) the
pars medialis of the posterior ventral nucleus receives vagal projections posteriorly; according to Aider
et al. (1952), the
pars lateralis of the same nucleus receives splanchnic projections in its dorsolateral region.

The interventral nucleus is one of the median-line nuclei which receives vagal projections (Dell and Olson, 1951).

In recent decades, it has also been shown that, at the level of subcortical and cortical centres, certain nuclei are able to receive vegetative impulses, some coming from the splanchnic nerve and others from the vagus.

Lastly, we shall refer to Erofeev's experiment, reported by Pavlov
[
6] . He succeeded in conditioning a dog, by associating the giving of food with a painful stimulation, causing feeding behaviour to take precedence over behaviour associated with pain (salivary secretion, chop-licking, etc. very quickly replacing reactions of flight and dilation of the pupils).

Before summarizing the effects on pancreatic secretion, of two factors modifying glucide metabolism, we should like to describe the technique used.

Preliminary work showed us that rats are suitable for research on pancreatic secretion, provided the care necessitated by the smallness of this ideal laboratory animal and the small quantity of digestive liquid collected is exercised.

The special structure of the rat's pancreatic ducts, which lead not to the duodenum but to the lower part of the choledoch, facilitates collection of a pure pancreatic secretion.

Male Wistar rats weighing approximately 300 g are anaesthetized by 1.20 g/k of ethylurethane administered intramuscularly after a 24-hour fast with no restriction on water intake. Thirty minutes after administration of the anaesthetic, the rat is placed on its back and attached by its paws to a wooden board. The temperature of the animal is automatically regulated and maintained at 38°C. The femoral vein is bared and catheterized with a polyethylene catheter.

The rat undergoes a twofold fistulization, biliary and pancreatic, by a technique already described
[
6] .

The biliary and pancreatic secretions are collected, the production of drops being recorded automatically. The length of the experiment varies between 10 and 24 hours.

As regards the pancreatic secretion, with which we are concerned here, the following measurements are taken: volume secreted; concentration and rate of discharge of protide, concentration and rate of discharge of amylase.

With normal rats, the rate of the pancreatic discharge first decreases slightly then gradually increases until it doubles by the 24th hour. The variation in protide concentration follows a course opposite to that of the rate of the pancreatic discharge, so there is a slight divergence between the rate of discharge of protides and that of the pancreatic secretion. Concentration and rate of discharge of amylase diminished until about the 8th hour, after which they remained more or less constant until the 24th hour.

In the experiments summarized below, the rat was left undisturbed for four or five hours after the operation, before the substance under study was injected, so that the injection could be made at a stage of pancreatic secretion that was more or less level. The various parameters were measured for one hour before (zero time) and four hours after the injection.

As we know, suffering may be connected with a homoeostatic anomaly such as hypoglycaemia
[
20] . Certain regions send a message to the brain, which in everyday language is called discomfort.

Following intravenous injection of 0.2 units of insulin (Novo) per rat, we observe a considerable increase in the production of pancreatic fluid and in the enzyme content of the pancreatic fluid (whether the total enzyme content is assessed in terms of the protide level or of the amylase content)
[
7] . The hydrelatic effect of the insulin does not reach a peak until the second hour after injection, at the moment when the hypoglycaemia is at its ceiling; in contrast, the ecbolic effect (protides and amylase) is at its peak one hour after injection.

When rats are injected with atropine before insulin, we find that the ecbolic effect is completely suppressed and the hydrelatic action very greatly reduced.

Under the same experimental conditions, we observe
[
9] that an intravenous injection of 25 mg of 2-desoxy-D-glucose (Calbiochem, quality A) per rat induces a remarkable increase in the production of pancreatic fluid with a sharp rise in its enzyme content. A difference between insulin and 2-desoxy-D-glucose is that with the latter the hydrelatic and ecbolic effects are parallel. Pancreatic output is at its peak during the first two hours, and the same applies to the concentration and output of protides and to amylase (where the results can be superimposed on those of protides).

Hirschowitz
[
10] points out the remarkable work of his team and their many confirmations of the effects of desoxyglucose through vagal excitation. This action is immediately blocked by the administration of atropine. The vagal action is attributable to the fact that 2-desoxy-D-glucose simulates hypoglycaemia at the level of the cerebral cells. This cytoglycopenic action is accompanied by hyperglycaemia, for desoxyglucose produces glycolysis inhibition through lack of hexokinasic activity following reduction of available ATP due to sequestration of phosphate by non-metabolizable 2-desoxy-D-glucose-6-phosphate.

Muller, Frohman and Cocchi, one of whose publications
[
11] we consider it useful to summarize, made a special study of the effects of 2-desoxy-D-glucose administered centrally. They examined the role of the autonomic nervous system in the hyperglycaemiating effect of desoxyglucose, which is accompanied not by increased release of insulin but, on the contrary, by its inhibition.

They show that this action originates centrally at the hypothalamus, whence impulses leave for the peripheral nerve effectors. The effects are produced via the suprarenals and the secretion of catecholamines. Blockage of the α-adrenergic receptors by intraventricular (right lateral ventricle of the brain) administration of phentolamine was shown to be incapable of preventing 2-desoxyglucose hyperglycaemia (centrally, phentolamine itself induces hyperglycaemia).

There is a dissociation between the hyperglycaemiating effects of desoxyglucose and its stimulating effects on food intake (the effect of phentolamine on the latter is to suppress them).

The intracerebral administration of propanolol failed to produce blockage of the β-adrenergic receptors.

Central interruption of the adrenergic system of the brain, induced by α-methyl-tyrosine, which inhibits biosynthesis of the catecholamines, did not modify the hyperglycaemiating effect of desoxyglucose (although it does alter its nutritional response). The same occurs after the destruction of the central catecholamines in the nerve ends by 6-hydroxydopamine.

A parasympatholytic, such as atropine, did not modify the hyperglycaemiating effect of desoxyglucose.

Phentolamine, administered (2 mg/kg/I/P) 15 minutes before desoxyglucose, blocked hyperglycaemia in the same way that suprarenalectomy does. A dose of 4 mg of phentolamine administered intraperitoneally also removed the inhibiting effect of desoxyglucose on the insulin secretion stimulated by glucose.

At all events, we elected to use 2-desoxy-D-glucose to study the possible effects of a central analgesic on pancreatic secretion; it was to provide us with a simpler model, since, unlike insulin, it does not dissociate its quantitative effects on the hydrelatic and ecbolic secretions.

It was first considered necessary to determine the possible effects of methadone on the pancreatic secretion of rats anaesthetized by means of urethane.

The results obtained after subcutaneous injection of rats with 5 mg/kg DL-methadone hydrochlorate (0.5 g dissolved in 100 ml of a 7 per mille sodium chloride solution) are shown in table I.

TABLE I

Effects of methadone on pancreatic secretion

(Average; standard deviation; groups of 10 rats)

Time (min)

- 60

+ 30

+ 60

+ 90

+ 120

+ 150

+ 180

+ 210

Pancreatic rate of discharge

(drops/hour)

3.00

±0.22

2.81

±0.21

2.81

±0.21

2.95

±0.20

2.92

±0.19

3.01

±0.24

3.17

±0.23

3.60

±0.29

Concentration of protides

(g/1 000)

20.10

±1.86

20.03

±1.50

19.54

±2.06

17.21

±1.79

16.62

±1.77

16.03

±1.80

13.50

±1.69

13.66

±1.54

Rate of discharge of protides

(µg/min)

10.34

±1.09

8.88

±0.86

9.06

±0.92

8.67

±0.84

8.60

±0.80

8.79

±0.94

8.95

±1.14

10.55

±1.14

Concentration of amylase

(U/mg)

7.91

±1.18

7.06

±0.79

6.75

±0.66

5.87

±0.71

5.95

±0.77

5.99

±0.75

5.171

±0.63

5.41

±0.66

Rate of discharge of amylase

(U/min)

4.29

±0.84

3.15

±0.43

3.11

±0.31

3.01

±0.38

3.01

±0.32

3.25

±0.43

3.37

±0.39

4.12

±0.43

A comparison of the effects observed after methadone with the measured secretion of control rats of the same origin reveals no significant differences.

In an earlier article
[
12] we showed that morphine and codeine increased the pancreatic rate of discharge of rats very slightly but the morphine and codeine were injected intravenously, the doses being 10 mg/kg for morphine and 20 mg/kg for codeine. Nor did those two analgesics significantly modify the concentration and rate of discharge of protides. Nicocodine, administered intravenously in a 10 mg/kg dose had no effect.

We have reported that methadone completely supresses the increase in pancreatic secretion induced by 2-desoxy-D-glucose
[
13] in the following conditions: at zero hour (i.e. five hours after the operation) the first batch (10 rats) are given an intrafemoral injection of 25 mg 2-desoxy-D-glucose per rat. Five minutes before an intravenous injection of the same amount of 2-desoxy-D-glucose, the second batch of rats receive a subcutaneous injection of 5 mg/kg of D-L methadone.

Variations in the rate of discharge of pancreatic secretion are shown in table II.

TABLE II

Effects of desoxyglucose alone (D) and in conjunction with methadone (D+M) on volume of pancreatic secretion

(Average and standard deviation, in drops per hour)

Time (min)

- 60

+ 30

-F 60

+ 90

+ 120

+ 150

+ 180

+ 210

D

3.14

±0.37

5.81

±0.66

7.42

±1.01

7.69

±0.75

6.65

±0.39

6.21

±0.45

6.10

±0.28

5.6l

±0.21

D + M

3.03

±0.22

2.89

±0.18

3.05

±0.27

3.43

±0.21

3.23

±0.29

3.35

±0.21

3.11

±0.20

3.33

±0.39

TABLE III

Effects of desoxyglucose alone and in conjunction with methadone on the protides and amylase of pancreatic secretion

(Average and standard deviation)

Time

- 60

+ 30

+ 60

+ 90

+ 120

+ 150

+ 180

+ 210

Log. concentration of protides

D

3.17

3.30

4.14

4.29

4.32

4.19

4.22

4.23

±0.16

±0.16

±0.18

±0.13

±0.13

±0.15

±0.17

±0.15

(g/1 000)

D +M

3.33

2.96

2.94

2.90

2.95

2.88

2.90

2.96

±0.17

±0.14

±0.15

±0.16

±0.17

±0.18

±0.19

±0.19

Log. rate of discharge of protides

D

2.49

3.33

4.41

4.64

4.54

4.41

4.49

4.31

±0.11

±0.25

±0.27

±0.22

±0.20

±0.24

±0.24

±0.18

(μg/min)

D + M

2.73

2.33

2.34

2.50

2.43

2.40

2.38

2.48

±0.15

±0.13

±0.09

±0.15

±0.11

±0.13

±0.14

±0.17

Log. concentration of amylase

D

2.15

2.25

3.07

3.12

3.12

3.00

2.95

2.98

±0.10

±0.14

±0.17

±0.12

±0.11

±0.11

±0.12

±0.12

(U/mg)

D + M

2.28

2.06

2.06

1.99

2.07

1.96

2.02

2.04

±0.16

±0.16

±0.15

±0.14

±0.15

±0.12

±0.14

±0.15

Log. rate of discharge of amylase

D

1.47

2.28

3.34

3.48

3.35

3.22

3.22

3.06

±0.12

±0.22

±0.24

±0.20

±0.16

±0.19

±0.15

±0.12

(U/min)

D + M

1.68

1.44

1.47

1.59

1.55

1.48

1.49

1.58

±0.14

±0.16

±0.11

±0.15

±0.10

±0.07

±0.08

±0.12

The rate of discharge of pancreatic fluid is thus significantly increased by desoxyglucose (at its peak the hydrelatic output is approximately 2.5 times greater), the increase persisting even four hours after the injection. Methadone completely suppresses this increase.

The variations observed in the enzymes of pancreatic secretion are given in table III. In order to reduce the apparent extent of the differences in concentrations and rates of discharge the results are presented in logarithmic terms.

At the maximum, desoxyglucose multiplies total protein concentration by approximately 3.5, and amylase concentration by approximately 3. The over-all rates of discharges of proteins and amylase are thus increased by a factor of the order of 8 to 10.

In order to see whether methadone does not counteract the effects of a centrally originating vagal excitation, at the level of the peripheral receptors, it was necessary to investigate the action of exogenous acetylcholine on pancreatic secretion with and without methadone.

We already knew the effects of acetylcholine from our experimental model
[
14] and we therefore selected an average dose for the rats. At zero hour (again five hours after the operation) the 10 rats of the first batch each received by venous (femoral) perfusion a total dose of 1 mg acetylcholine in a total volume of 4 ml over a period of 25 minutes; five minutes before the perfusion, the rats of the second batch were given a subcutaneous injection of DL methadone hydrochlorate.

The results, as regards the rate of discharge and concentration of enzymes in the pancreatic fluids, are shown in table IV. In order to simplify presentation, we have not included in the table the standard deviations which, in this particular case, were of no interest; statistical treatment of the results showed that there was no significant difference between the two batches of animals.

The batch of rats given only acetylcholine started with a lower rate of hydrelatic discharge but, as regards its effects, the action of acetylcholine administered in conjunction with methadone is identical. In so far as amylase and total proteins are concerned, however, the rats given only acetylcholine started with a higher concentration, but this does not prevent obtaining the same effects on the ecbolic secretion of the pancreas when methadone is present.

Moreover, as we have shown
[
14] , acetylcholine probably does not act at the ganglionic level, because it maintains its effects on pancreatic secretion in the presence of a ganglion-blocker, such as spartein. In contrast, atropine suppresses the ecbolic effect of exogenous acetylcholine by its post-ganglionic action of saturating the reactive capacity of the receptors
[
14] .

It may be inferred from this that methadone does not exert an anticholinergic effect at the level of the pancreas itself.

In order to determine the specificity of the effects of methadone on secretion induced by centrally originating vagal excitation, it was necessary to investigate the effects of such morphinic antagonists as nalorphine
[
15] or naloxone.

TABLE IV

Effects of acetylcholine alone and in conjunction with methadone on the pancreatic secretion of rats

(Average : groups of 10 animals)

>(Acetylcholine = A: Methadone + Acetylcholine = A + M)

Time

- 60

+ 30

+ 60

+ 90

+ 120

+ 150

+180

+ 210

Rate of pancreatic

A

2.30

9.11

4.99

3.99

3.46

3.26

3.36

3.40

discharge (drops/hour)

A + M

4.01

12.05

6.01

5.40

5.40

5.39

5.14

5.11

Log. concentration

A

3.44

4.22

4.38

3.91

3.58

3.32

3.40

3.38

of protides (g/1000)

A + M

2.87

3.75

4.15

3.17

2.80

2.70

2.70

2.64

Log. rate of discharge

A

2.50

4.69

4.40

3.53

3.10

2.83

3.03

3.02

of protides (g/min)

A + M

2.58

4.51

4.25

3.17

2.79

2.67

2.65

2.59

Log. concentration

A

2.19

2.88

2.97

2.46

2.27

2.23

2.1 1

2.09

of amylase (U/mg)

A + M

1.89

2.64

2.92

2.06

1.75

1.67

1.74

1.63

Log. rate of discharge

A

3.50

3.36

2.85

2.08

1.79

1.75

1.73

1.74

of amylase (U/min)

A + M

1.80

3.39

3.03

2.07

1.73

1.64

1.58

1.58

The experiment with nalorphine hydrochloride is summarized in table V. The substance is injected subcutaneously in an amount of 3 mg per rat. We then administer, subcutaneously, 5 mg/kg of methadone in the manner reported above and, five minutes later, intravenously, 25 mg of desoxyglucose per rat.

TABLE V

Effects of desoxyglucose in conjunction with nalorphine and methadone

Time

- 60

+ 30

+ 60

+ 90

+ 120

+ 150

+ 180

+ 210

Rate of pancreatic

2.78

3.94

5.97

5.75

5.70

4.73

4.31

4.09

discharge(drops/hour)

±0.22

±0.48

±0.70

±0.66

±0.77

±0.72

±0.60

±0.60

Log. concentration

3.67

3.47

3.84

4.34

4.46

4.30

4.20

3.95

of protides(g/1000)

±0.17

±0.15

±0.19

±0.14

±0.14

±0.17

±0.17

±0.19

Log. rate of discharge

2.87

3.03

3.81

4.34

4.39

4.01

3.83

3.54

of protides(μg/min)

±0.18

±0.20

±0.28

±0.21

±0.21

±0.20

±0.24

±0.27

Log. concentration

2.26

2.22

2.48

2.88

3.04

2.85

2.68

2.49

of amylase (U/mg)

±0.15

±0.14

±0.21

±0.14

±0.16

±0.19

±0.19

±0.20

Log. rate of discharge

1.47

1.78

2.45

2.87

2.96

2.55

2.31

2.08

of amylase(U/min)

±0.19

±0.20

±0.29

±0.23

±0.22

±0.25

±0.28

±0.29

A statistical comparison, made by means of the Tukey test, of the results thus obtained with those produced by desoxyglucose alone, reveals no significant differences. Thus, the nalorphine counteracted the effects of methadone on desoxyglucose-induced pancreatic secretion.

It was essential to investigate the effects of tolerance to methadone
[
16] . The following is a summary of the experiment.

For a period of two or four weeks, batches of Wistar rats are given a daily (except Sundays) subcutaneous injection of increasing doses of DL methadone hydrochloride as follows: 5 mg/kg for five days; 7.5 mg/kg for three days; 10 mg/kg for three days; 12.5 mg/kg for six days; and 15 mg/kg for seven days.

Twenty-four hours after completion of the treatment, the animals are anaesthetized with urethane and subjected to a double biliary-pancreatic fistulization (6). After establishing the basic level of the secretion, we inject, subcutaneously, 5 mg/kg of DL methadone hydrochloride, and five minutes later, intravenously, 25 mg of 2-desoxy-D-glucose per rat. The results obtained are compared with those of batches given either 2-desoxy-D-glucose alone or methadone plus 2-desoxy-D-glucose, without prior chronic treatment.

The results obtained after two weeks of treatment are summarized in Table VI, which does not show standard deviations or the results obtained with amylase which faithfully concord with those observed for total proteins.

The results obtained after four weeks of treatment are given in able VII; to facilitate comparisons, we have also shown the results obtained after injecting desoxyglucose into control rats not treated with methadone.

In rats that have been rendered methadone-tolerant for two weeks, a strong injection of methadone further depresses the rate of pancreatic discharge and delays the effect of desoxyglucose. The effect is significantly reduced for one-and-a-half hours after the injection of desoxyglucose.

TABLE VI

Effects of desoxyglucose after methadone following chronic treatment for two weeks

Time

- 60

+ 30

+ 60

+ 90

+120

+ 150

+ 180

+ 210

Rate of pancreatic discharge (drops/hour)

3.13

3.85

4.22

4.55

5.11

4.85

5.35

6.33

Log. concentration of protides (g/1000)

4.28

3.96

4.81

4.76

4.75

4.65

4.71

4.77

Log. rate of discharge of protides (μg/min)

3.36

3.49

3.40

3.55

3.63

3.59

3.72

3.99

TABLE VII

Effects of desoxyglucose after methadone following chronic treatment for four weeks

After four weeks of methadone treatment, a strong injection of methadone becomes incapable of inhibiting desoxyglucose stimulation. It is even significantly increased one and a half hours after the injection of desoxyglucose, as compared with the control rats given desoxyglucose only.

We noted, further, that at the commencement of pancreatic-fluid collection (i.e. four hours before the time selected as zero hour for the injection of 2-desoxy-D-glucose) there was hypersecretion of pancreatic fluid in animals which had received chronic treatment with methadone, ranging from 9.1 ± 2.12 for those treated for two weeks and 18.9 ± 3.49 for those treated for four weeks. This hydrelatic hypersecretion diminished rapidly and one hour before the desoxyglucose injection reached a base level very comparable to that of the control animals.

The concentration and base rate of discharge of the proteins and amylase of pancreatic fluid are much higher than in rats not chronically treated with methadone, both after two and four weeks of treatment (P<0.01 to < 0.001).

To a certain extent, this increase is to be compared with the protide-concentration results we obtained by anaesthetizing the rats with mebubarbital [66] instead of urethane.

Contrary to what occurs with new animals given 2-desoxy-D-glucose only, protein concentration does not vary under the effect of a strong injection of methadone and 2-desoxy-D-glucose.

The rates of protein and amylase discharge vary as the rate of discharge of pancreatic fluid: the effect of 2-desoxy-D-glucose is weak and delayed in rats which have been habituated for two weeks; from 30 to 180 minutes after being given 2-desoxy-D-glucose, their rate of discharge is lower than after stimulation by 2-desoxy-D-glucose in new animals. In the rats habituated over a period of four weeks there is a definite increase in ecbolic discharge even though they start from a higher base level; the enzyme levels reached are identical with those observed in new animals.

Thus, under the effect of 2-desoxy-D-glucose, the spectacular increase in the external pancreatic secretion of rats can be suppressed by prior administration of methadone. The inhibition relates to both the hydrelatic and the ecbolic secretion.

This effect is not due to a peripheral blockage of the cholinergic receptors, because the pancreatic effects of exogenous acetylcholine are maintained in the presence of methadone whereas atropine suppresses it.

Nalorphine counteracts the effects of methadone on pancreatic secretion induced by 2-desoxy-D-glucose. Chronic treatment with methadone produces two types of phenomena: a desensitization - partial after two weeks and complete after four weeks in respect of the rate-of-discharge parameters - to blockage of the effects of 2-desoxy-D-glucose by a single injection of methadone; and a basal pancreatic hypersecretion in terms of both water and enzymes.

Surveillance of the pancreatic secretion thus makes it possible to envisage a promising experimental model for the study of central analgesics.

Would it not also be possible, with this model, to clarify the hypotheses of research workers who for decades have been studying the workings of the central analgesics?

A certain consensus is emerging concerning the anatomo-physiology of pain [17, 18, 19]. In the central make-up of the latter, a distinction should be made between direct central mechanisms and indirect, delayed-action multisynaptic mechanisms. Laborit
[
19] describes this latter central action inhibiting the sensation of pain and cites the work of Denise Albe-Fessard (1966), according to whom caudate and pallidal stimulation (at the level of cerebral functional areas) passing through the reticular formation, activates the inhibition controlling the sensation afferents. Thus, this stimulation inhibits the potential reactions to stimulation of cutaneous nerves, in the reticular formation, the intralaminar nuclei of the thalamus and the non-specific cortex. Laborit also refers to the work of Melzack and Wall (1968) who consider that the inhibiting action of central origin operates at the level of what they call the "central gate" system ("gate-control" theory of pain).

According to Quarti and Renaud
[
20] pain "results from the discharge of specific structures of the mesencephalic reticulum in three directions: towards: the thalamocortical couple, the hypothalamus and the spinal motor element".

They add : "There are many objective arguments in favour of thinking that pain is not just another form of sensation but a vital system of relating to the world and to oneself that is essential to survival ... Since it necessitates the participation of the whole central nervous system, there can certainly be no question of doing away with it altogether, Nevertheless, although the entire organism's reactions to pain cannot be prevented, it is possible to contemplate eliminating a certain consciousness of pain, namely the appreciative consciousness."

It is accepted that the mechanisms that integrate pain operate at a level prior to any cortical afference, which latter permits the representation of pain, although unlike the case of other perceptions (for example, visual) it has not been possible to demonstrate a cortical localization of pain afferences.

Histochemistry has made it possible to detect adrenergic neurons not only throughout the neuraxis but with a main iocalization in the brain stem, the hypothalamus and the limbic system. Arousal with electrocortical activation is produced by the intracarotid injection of noradrenaline: this activation is attributable to adrenergic receptors situated in the reticular formation of the brain stem and the hypothalamus, without any direct action on the cerebral cortex.

Histochemistry has also helped to localize serotonin along the median line of the brain stem (raphe system): serotonin apparently constitutes the basis of a system functioning as antagonist of the lateral reticular formation and possibly provokes sleep by acting on structures of the neopallium. Serotonin is also found in various structures of the limbic system. Serotonin's main role would seem to be that of a modulating substance. Soulairac lays stress on the extensive system of ascending cholinergic fibres "which may be termed an ascending cholinergic reticular system, with projection to the neocortex and the subcortical regions, mainly the corpus striatum and the hyppocampus". Cholinergic activity has been demonstrated in the intralaminar nuclei and the reticular structures of the thalamus, excluding the specific nuclei of the thalamus.

As A. Herz emphasizes (in 18), it may seem paradoxical that the places where analgesics act on the central nervous system should still be the subject of debate. The intense action of morphine when injected into the ventricular system would indicate that it acts at the level of the periventricular structures (Horlington and Lockett, 1955, in 18).

In the opinion of Teschemacher
et al. (in 18), inhibition of the nociceptive reaction by morphinic analgesics is significant mainly in the region of the aqueduct and the fourth ventricle. Tsou and Jang (1964, in 18) contend that the place in question is in the region of the third ventricle.

These discrepancies are perhaps attributable to the use of different nociceptive methods.

Lastly, the possibility of a diffusion of the analgesic through the ventricular system cannot be excluded.

In the opinion of Albus
et al. (in 18), a study of such antagonists as levallorphan and nalorphine shows that, after introduction into the ventricular system, they completely reverse the action of morphine at the level of the caudal parts of the ventricular system, particularly the fourth ventricule, and the adjacent segments of the brain stem.

By injecting morphine in various subcortical parts of the brains of rats, Jacquet and Lajta [21 ] observe the occurrence of significant analgesia in the case of the posterior hypothalamus, whereas the same dose of morphine injected into the medial septum, the caudate nucleus or the peri-aqueductal grey matter causes hyperalgesia (with violent stereotyped circular jumps).

Clouet and Williams
[
22] administered radioactively traceable morphinic analgesics and antagonists to rats before killing them. The morphinic substances were localized in the fraction of the brain containing the pinched pre-synaptic nerve ends, the synaptosomes, and in the soluble part of the tissue. The antagonists were localized in the same particulate fraction of the brain.

There has been considerable evidence of the role played by the hypothalamus in gastro-intestinal regulation.

As far back as 1931, La Barre and Cespedes guessed that the action of hypoglycaemic substances favourable to gastric secretion was attributable to activation of the brain's vagal centres. Direct proof was, however, provided by the injection of 2-desoxy-D-glucose into the lateral cerebral ventricle
[
23] or the hypothalamus
[
24] of rats.

Other experiments
[
25] suggests that the lateral hypothalamic region, and the ventromedian nucleus of the hypothalamus contain cells sensitive to glucose.

Other research, using the hypoglycaemic effect of insulin, instead of the cytoglycopenic action of 2-desoxy-D-glucose, to provoke gastric secretion, have given similar results. In addition, Pokryshkin
[
26] injected insulin in various nuclei of the hypothalamus of the cat and recorded the bio-electrical activity of different parts of the brain. He attributes the changes observed in the electro-encephalogram of the limbic system and the neocortex of the large hemispheres to an activating influence-apparently of a cholinergic type-of the hypothalamus.

Jacob and Barthelemy (in 18) summarize the work done on the relationship between the anti-nociceptive effect of morphine and central cholinergic systems and emphasized that interpretation of the modifications observed is still very much a matter of debate.

Many writers showed that narcotic analgesics increased the total quantity of acetylcholine in the brain of rats and mice. Closer studies, however, revealed that the increase was not uniform. By perfusing the ventricles of the cat's brain, Beleslin and Polak
[
27] discovered that morphine somewhat reduced the quantity of acetylcholine in the effluent. Jhamandas
et al.
[
28] note that spontaneous release of acetylcholine from the cerebral cortex of a living cat is diminished by morphine, but not by nalaxone.

Using different techniques, Domino
et al. [29, 30] developed this research considerably and proved that, in the rat, morphine reduces cortical release of acetylcholine. The agonistic action of nalorphine in sufficient doses produces a more prolonged inhibiting effect of acetylcholine release in the cat than in the rat. In contrast, naloxone does not have this effect and counteracts the effect of morphine.

Laborit
[
19] points out that the relationships between analgesic action and the metabolism of cerebral catecholamines has been the object of much research.

In many animals, morphine releases dopamine and noradrenaline in the brain within an hour following injection.

Sympathomimetic agents, which, like morphine, release reserves of catecholamines would seem to have analgesic effects.

Turnover of catecholamines, and also of serotonin, is increased by narcotic analgesics.

In consolidating the disparate experimental data, Laborit concluded that "analgesia would seem to be linked to increased serotonin and noradrenaline turnover, probably in conjunction with central cholinergic release, whereas dopamine accumulation seems to favour algetic phenomena".

As Boissier and Baum
[
31] have observed, however, the extrapyramidal cholinergic system should not be reduced simply to the dopamine-acetylcholine antagonism of the striatal system.

Lastly, it must be remembered that pain itself is responsible for a defensive process involving in particular the release of medullo-suprarenal catecholamines through the hypothalamus.

In conclusion, we shall cite M. Weinstock who, in her work with B. M. Cox, showed that there was a very significant correlation between the analgesic capacity of narcotic analgesics and their capacity to inhibit acetylcholine release from the isolated ileum of the guinea pig. She states
[
32] that if some close relationship could be found between analgesic capacity and inhibition of release of acetylcholine from the central neurones, together with proof that nalorphine and the chronic administration of morphine also attenuate the effect, scientists would be well on the way to understanding how morphine acts in the central nervous system.

According to Goldstein
et al.
[
33] , the mechanism whereby tolerance to, and physical dependence on, narcotic drugs is developed is as yet unknown, although there is no lack of speculative theories. At present, there are two categories of theories: one postulates a continuous and unchanged interaction between the drug and its receptors, the effects of the former being counteracted or compensated by changes in other biochemical processes or in other neurone systems; the other attributes tolerance to a change in the drug receptors themselves, in the form of either a modification in their numbers or a change in their properties making them less sensitive to the drug. In the excellent synthesis by Goldstein
et al., of the work done on this question, we draw attention to the passage in which the authors state that a more concrete version of a theory of the first category postulates that the post-synaptic receptors for an endogenous neurotransmitter become hypersensitive (Collier, 1966 and 1969) as in the well-known phenomenon of denervation hypersensitivity (Jaffe and Sharpless, 1965); and that, after denervation of the effectors innervated by the sympathetic nerve in the peripheral autonomic system, the effectors become hypersensitive to noradrenaline, the main reason for this being loss of the uptake and storage capacity of noradrenaline, which is ordinarily associated with the terminal extremity of the adrenergic nerve and plays an important role in stopping the effects of noradrenaline. According to the authors, it is believed that a long period of depression of the brain by narcotics, barbiturates or alcohol may cause a sort of functional denervation of the central links, so sensitizing them that they react above their normal level when the drug is suppressed (abstinence), and that this increased activity of the sensitized links antagonizes the effects of the drug.

According to W.R. Martin
[
34] , pharmacological redundancy would seem to offer an explanation of the adaptive mechanism of the central nervous system. He recalls that Himmelsbach (1943) was the first to suggest that chronic administration of morphine was followed by stimulation of contra-adaptive forces which, by antagonizing the morphinic effects, produced tolerance and dependence.

In support of that theory, he advances three series of experimental results which he summarizes as follows:

The depression of certain functional links produced by heavy doses of neurohumoral antagonists may be overcome by increasing the strength of the stimulus. As the quantity of neurohumoral transmitter which may be released by the pre-synaptic element is limited and would not be expected to overcome the depression produced by strong doses of the antagonist, a neuronal shunt around the depressed synapse has been suggested.

The redundancy theory calls for continuous activity of the drug if there is to be hypertrophy of the redundant link and development of tolerance and physical dependence. Several cases present evidence that the degree of morphine-like dependence is linked to the strength of agonist activity and that the pharmacological activity of morphine persists in the resistant animal.

Lastly, four distinct pharmacological mechanisms (cholinergic, adrenergic, tryptaminergic and 5-hydroxytryptophane-like) have been identified which facilitate the flexion reflex and induce the walking reflex, both of which are signs of morphine dependence and deprivation. Although none of these mechanisms is probably essential, or even necessary, to the development of morphine dependence, their existence is compatible with the hypothesis of the existence of pharmacological redundance in the spinal cord.

E.
Our contribution to the foregoing hypotheses

Methadone inhibition of central vagal pancreatic stimulation provoked by 2-desoxy-D-glucose, its inversion by nalorphine and its attenuation after chronic treatment satisfy Marth Weinstock's three imperatives.

The localization - in the central nervous sytem - of narcotic analgesics and such cytoglycopenic substances as 2-desoxy-D-glucose and insulin allow one to suppose that the former interfere in the cholinergic action of the latter. It is probably at the level of the hypothalamus that the analgesic, interrupting transmission to the cortex, suppresses consciousness of pain.

It is further possible that the other effects of the analgesic on the cerebral amines play a part by augmenting the central action inhibiting the transmission of painful sensations.

Basal pancreatic hypersecretion after chronic intoxication supports one of the above-mentioned theories on tolerance to narcotic analgesics, Its mechanism has, however, yet to be defined. It may be a hypersecretion due to hyperfunctioning of the vagal secretory neurons, which are known to be capable of inducing very prolonged secretions (in 16). On the other hand, it may be an experimental artefact deriving from the anaesthesia, which induces a heavy discharge of catecholamines.

In conclusion, we wish to observe that instead of basing ourselves on a study of the pancreatic secretion to test the central action of analgesics, we could have investigated the gastric secretion. For results similar to those obtained on the pancreas, reference should be made to the work of D.A. Brodie
et al.
[
35] . Studying the effects of the injection of various psychotropic substances into the cerebral ventricle of the rat, they observed, inter alia, that morphine reduces the effects of stress (caused by restraint plus cold) on the gastric secretion of the rat. The Jeanne Levy school
[
36] had also shown that dextromoramide, in particular, reduces stress-related gastric ulcers in rats.

Charles R. Schuster and Chris E. Johanson - The use of animal models for the study of drug abuse, pages 1-31 - Research advances in alcohol and drug problems, John Wiley and Sons, New York, London. Vol. 1, 1974.